Electrical Switchboards – Overcurrent protection – overload, short circuit, discrimination and cascading

Overload protection

Overload Protection

Overload protection is essential in safeguarding electrical conductors from the thermal effects caused by currents exceeding their rated capacity. When a conductor is subjected to a current higher than its nominal rating but not high enough to qualify as a short circuit, it risks overheating. This condition can lead to insulation damage, equipment failure, or even fire. Overload protection mechanisms are designed to detect and interrupt these excessive currents before they cause harm.

For example, consider a 630A circuit breaker. If this breaker experiences a current of 650A, it might take approximately one hour to trip. However, if the current increases to 800A, the breaker will trip in about one minute. This varying tripping time is a characteristic of overload protection, where the trip time is inversely related to the magnitude of the overcurrent. This relationship is often represented by an IDMT (Inverse Definite Minimum Time) curve. The IDMT curve indicates that the higher the overcurrent, the quicker the breaker will trip, but the exact time is not fixed, hence the term "indefinite."

Traditionally, overload protection is provided by the thermal component of a thermal-magnetic circuit breaker. The thermal element, typically a bimetallic strip, heats up as current flows through it. When the current exceeds the breaker's rating, the strip bends and triggers the trip mechanism. However, modern systems may also use electronic circuit breakers or protection relays equipped with current transformers (CTs) to measure the current. These devices can be programmed to provide overload protection with precise control over trip settings.

When designing an electrical protection scheme, it is ideal to ensure that only the smallest possible section of the electrical system is disconnected during an overload event. This selective tripping minimizes disruption to the overall system. In some cases, definite mean time (DMT) protection is employed using protection relays. In this approach, if the current exceeds a certain threshold, the relay will trip after a predetermined amount of time, providing a more predictable response compared to IDMT.

Discrimination Between Electrical Switchboards and Circuits

Discrimination, also known as selectivity, is a critical concept in electrical protection. It ensures that only the faulty section of an electrical system is disconnected during an overload or short circuit event, leaving the rest of the system operational. Achieving discrimination requires careful coordination between the trip curves of circuit breakers.

Discrimination studies are performed to compare the trip curves of various circuit breakers in a system. The goal is to minimize the overlap between the trip curves, ensuring that the circuit breaker closest to the fault trips first, while upstream breakers remain closed. This selective tripping is particularly important in complex electrical systems with multiple levels of protection. By preventing unnecessary tripping of upstream breakers, discrimination helps maintain power supply to unaffected areas of the system, reducing downtime and enhancing system reliability.

Short Circuit Protection

Short circuit protection is designed to prevent catastrophic failures in electrical systems. Unlike overload protection, which deals with moderately excessive currents, short circuit protection addresses extremely high currents that occur when a fault causes a direct connection between conductors. These faults can produce currents many times higher than the system's nominal rating, posing an immediate threat to equipment and safety.

In thermal-magnetic circuit breakers, the magnetic element provides short circuit protection. When a short circuit occurs, the resulting surge in current creates a strong magnetic field that rapidly trips the breaker, often in less than one cycle (20 milliseconds in a 50 Hz system). This quick response is crucial to limiting the energy released during the fault, thereby reducing the risk of damage.

Short circuit protection is typically activated at around 10 times the nominal current. For instance, a 100A breaker might trip if the current exceeds 1000A. In the event of a short circuit, it is common for upstream breakers to trip as well, ensuring that the fault is isolated as quickly as possible. This protection can also be provided by protection relays and suitably rated protection CTs, which are designed to handle the high fault currents. It is important to note that metering CTs, which are used for measurement purposes, are not suitable for short circuit protection as they may become saturated and fail to relay accurate fault current information to the protection relay.

Cascading Protection

Cascading protection is a technique used by circuit breaker manufacturers to enhance the fault-breaking capacity of their products. In a cascading arrangement, a downstream breaker and its upstream counterpart work together to interrupt a fault. This allows the downstream breaker to clear faults that exceed its individual fault rating by relying on the upstream breaker to assist in fault interruption. Both breakers are expected to trip together, thereby sharing the fault-breaking duty.

Cascading protection is advantageous in systems where space or budget constraints limit the use of high-rated breakers throughout. By carefully selecting and coordinating breakers, designers can achieve robust protection without oversizing every component. This approach not only enhances safety but also improves the overall cost-effectiveness of the electrical system.